Just re-read what you wrote, and I noticed that you did say that you used the lineart tool. So, I had to re-think my answer. Line art will always look pixelled when you zoom in. I think you might be worrying a little too much about it.
Have you tried using the vector drawing tool in Sai yet? It really helps to clean up the line art. Yes, it means going over your lines again after you drew them, but it also means that you get a chance to improve your hand-eye coordination.
My best advice for lineart is to try different tools until you find one that suits you. As for me, I realized that when I used brown instead of black when doing my lineart, it went much better. It sounds strange, but I guess that everyone finds their tricks with time.
However, if you have Skype, I could sit down with you and chat for a while - Showing some things that I have learned. Maybe you will find something useful. Please just send me a note if you are interested.
Good luck anyway! Best way of improving is practicing!
Are you using photoshop? SAI is better for lineart, and also i would recomment drawing by hand, as it's usually smoother and looks more natural and nicer (at least in my opinion)
Theres a .7px brush on sai, and i can get thin and super small details in with a canvas half your size, so if you're not usuing already, i really recommend it.
Also, just something i realize (or maybe its a result from my paranoia) on photoshop (more often than sai), if you save, close and open it a lot, random 'dirt' pixels will show up (well it usually is just one random pixel that floats in random space, but it bothers me), so either keeping it open, or drawing lineart all in one go is probably better if you want REALLY clean lineart.